Argonne is initially baselining gasoline and ethanol at stoichiometric air/fuel ratios, and has begun expanding to other renewable fuels such as butanol. In April, the researchers presented a paper comparing gasoline and gasoline/ethanol blend combustion at the SAE World Congress in April, and a paper comparing ethanol and butanol as oxygenates at the ASME Internal Combustion Engine Division 2008 Spring Technical Conference.

The basic engine for all tests was a spark-ignited, direct injection Opel 2.2L Ecotec Engine (GM L850) with the cylinder head modified for cylinder pressure transducers. The stock engine operates with a compression ratio of 12:1 and delivers 114 kW of power @ 5600 rpm and maximum torque of 220 Nm @5600 rpm.

The engine control unit (ECU) controls engine operation parameters based on sensor inputs as well as driver demand. The sensors available on this particular engine setup are:

• Crank angle position
• Camshaft position
• Fuel rail pressure
• Engine coolant temperature
• Knock
• Throttle position feedback
• Exhaust oxygen content before catalyst
• Exhaust oxygen content post catalyst
• EGR valve position
• Swirl valve position
• Intake air mass air flow

The ECU calculates engine operational setting based on input from the sensors and driver demand using equations and look-up tables. The main output functions of the ECU are:

• Fuel injection timing
• Fuel injection duration
• Fuel injection pressure
• Ignition timing
• Exhaust gas recirculation (EGR) rate
• Swirl plate position

In the SAE paper, the Argonne team used the stock calibration of the injection parameters (fuel pressure and injection timing), nor were the position of the swirl control valve or the EGR rate changed when the engine was operated on the different ethanol/gasoline blends. The intent of the first study was to assess the effects on combustion behavior, efficiency, emissions, and performance of various blends of gasoline and ethanol without any adjustment of calibration parameters, on an engine calibrated for gasoline.

Fuels tested against a baseline fuel of pure gasoline were blend ratios of 10% (E10), 20% (E20), 50% (E50), and 85% (E85).

Comparison of brake thermal efficiency. Click to enlarge.

Results from this work showed that in a wide range of low- and medium-load operation,  neat gasoline and blends of ethanol and gasoline (E10, E20, E50 and E85) show very similar results in terms of brake thermal efficiency, with only high engine loads and blends with high ethanol content showing significant advantages in brake thermal efficiency. Other findings from this study included:

• The combustion behavior expressed as combustion speed, combustion stability, rates of heat release and combustion phasing do not show significant differences among the different blends of fuel.

• Blends with higher ethanol content do show advantages in terms of regulated emissions, both for oxides of nitrogen (NO_x) as well as total hydrocarbons (THC). These advantages are mainly due to the fuel composition such as lower aromatic content as well as the higher latent heat of vaporization.

The testing confirmed that:

The fact that ethanol allows for a more optimal combustion phasing at high engine loads indicates the necessity to design and calibrate an engine specifically for ethanol operation. This would permit taking full advantage of the promising combustion properties of ethanol and compensate for the lower volumetric energy content of ethanol compared to gasoline.

The ethanol-butanol study reported in the ASME paper analyzed the combustion, efficiency and emissions of pure gasoline, 10% ethanol (E10) and 10% butanol (Bu10) blends in the 2.2L engine. Major findings of this study include:

• The brake thermal efficiency was very similar between the three fuels and the peak values differed by less than 2% relative.

• Combustion analysis showed that before the onset of knock (load less than 75 Nm), the locations of start-of-combustion and peak pressure were identical for the three fuels. The magnitude of peak pressure was the highest for the Bu10 blend, the 50% MFB point was the most advanced and the maximum rate of pressure rise was highest for the Bu10 blend. This suggests the burning velocity of the Bu10 blend was higher than both the E10 blend and gasoline.

• The E10 blend had the highest octane rating which allowed the spark timing to be advanced up to five degrees compared to gasoline at the highest load of 150 Nm. At high loads, the ECU retarded the ignition timing to prevent knock with pure gasoline and the Bu10 blend, due to the lower octane rating compared to ethanol. Once knock occurs, the magnitude of peak pressure drops measurably for the Bu10 blend and the peak occurs several degrees later than gasoline or E10.

• Brake specific volumetric fuel consumption, a normalized measure of engine fuel consumption, showed an increase of approximately 3.4% for Bu10 and 4.2% for E10 compared to gasoline. Gasoline had the lowest BSVFC of the three fuels (311 ml/kWh) due to the high energy density.

• Combustion stability did not vary significantly between the three fuels, over the tested speed and load range. COV of IMEP was less than 3% for the entire operating range.

• Specific carbon monoxide emissions did not show a significant difference between gasoline and the two fuel blends. This was not an unexpected result due to the closed-loop feedback on the engine, to maintain stoichiometric operation.

• Specific THC emissions did not show significant differences between the three fuels, due to the formation and oxidation mechanisms not being dramatically altered for the three fuels. It is expected that as alcohol blend ratio increases, the impact on THC emissions should be positive due to the reduction in aromatic content in the blended fuel.

• Specific NO_x emissions reflected the ignition timing changes due to knock. The E10 blend produced the highest specific NO_x emissions and largest peak NO_x island, due to the advanced ignition timing. The lowest specific NO_x emissions were for Bu10, which had the largest ignition delay at the high load conditions due to the low octane rating of pure butanol.

The data suggests that 10 Vol-% butanol can be substituted for ethanol as an effective oxygenate, with an improvement in fuel economy and no degradation in emission or combustion stability.

Resources

• Thomas Wallner and Scott A. Miers (2008) Combustion Behavior of Gasoline and Gasoline/Ethanol Blends in a Modern Direct-Injection 4-Cylinder Engine (SAE 2008-01-0077)

• Thomas Wallner, Scott A. Miers and Steve McConnell (2008) A Comparison of Ethanol and Butanol as Oxygenates Using a Direct-Injection, Spark-Ignition (DISI) Engine (ICES 2008-1690)